A Plumbing union is used to make it easy to disconnect two sections of pipe. Soldering the union shut would defeat this purpose, and it would be more economical to use an ordinary coupling if there is no intention of disconnecting the pipes.
It is used to shut off (isolate) the flow of water from a plumbing fixture.
Shut off the water supply, get a fire extinguisher and then disassemble the union and unsweat it
NO. Its location will determine if it and its associated plumbing will freeze up.
Roughed-in plumbing refers to plumbing (water and waste lines) installed for future fixtures, for example, a bathroom in a basement. Stops refer to shut-off valves on water supply lines, required to shut-off the water to repair the fixture that they service. So "Rough-in stops" are shut-off valves on supply lines that are roughed-in. If they were not roughed-in, you would need to break concrete floor (to install waste lines) if/when you decided to add a bathroom to your basement family room.
No. The Main is source of water for that as well.
no they do not they exist all the time
First off, you should shut off the water source. Then you must drain the pipe. After that you can clean up the area to be soldered, apply some flux. Now you are ready to solder. The pipe doesn't need to be bone dry, since it will dry as you solder from the heat.
A friend should shut up when he/she does not have anything meaningful to say. Talking out of point can be hurtful.
A valve in a plumbing line that closes the line before the fixture it is connected to. For water there are usually shut off valves under the sink and right above the floor for the toilet. For gas, there is usually a valve that a flexible line attaches to and then to the appliance.
Yes
sounds like you have leaky valves, which need to be replaced, by someone with a plumbing background, not necessarily a licensed plumber.
Yes. But, only if the union is after a main shut off valve and is before and appliance connection. ex. A ball valve then pipe and a union before an furnace connection. Unions used as couplings to lengthen pipe are not code approved.